In the production of pulp by the sulfite process a feed material consisting of wood in most cases beech or fir or, in other geographic regions, consisting of a material such as bagasse, straw or stalks of corn (maize) and sunflowers, is chopped to form chips, which are treated in a digester with an acid cooking liquor which contains magnesium bisulfite and/or calcium bisulfite under a superatmospheric pressure and at temperatures up to 140.degree. C. for several hours.
In that digestion process, the lignin is sulfonated and the hemicellulose is hydrolyzed. The resulting magnesium lignosulfonates and/or calcium lignosulfonates and the hydroxylated hemicellulose are soluble and are separated as the so-called dilute liquor from the pulp.
The dilute liquor contains about 10% solids and is neutralized and is subsequently evaporated in a multiple-effect evaporating system to form a concentrated liquor, which contains more than 50% solids and is subsequently burned. The sulfur dioxide which is released and, in the magnesium bisulfite process, the magnesia recovered are recycled to the acid cooking liquor.
In the operation of evaporating plants for evaporation dilute liquor from the sulfite process, difficulties are encountered owing to the deposition of substances which are contained in the dilute liquor.
In the processing of annual plants, these deposits consist of silicates. Where wood is used as a raw material, deposits of gypsum must usually be expected.
The formation of deposits will be small if the evaporation is effected in multiple-effect vacuum evaporators at relatively low temperatures. But, in practice, multiple-effect evaporators in a hybrid arrangement are usually operated at elevated temperatures and under superatmospheric pressure.
The tendency to crusts can be decreased by having the steam and liquor flow paths periodically interchanged in the heat exchangers so that the calcium sulfate which has been separated will be washed out by the condensate in the next following cycle of operations.
In another process, the evaporators are operated at high velocities of flow and the spent liquor is preheated to precipitate a large part of the gypsum. While these measures will reduce the tendency to form crusts, they cannot entirely prevent a formation of gypsum deposits, which will seriously disturb the operation of the plant.
For this reason, various efforts have been made to prevent the formation of such deposits or to reduce the rate at which they are formed or to change their formation so that the cost and frequency of the operations to clean the several evaporating stages during the process can be reduced.
In the process which is known from Japanese Published Patent Application No. 78/55,490 ("Chem. Abstr.", Volume 89, Ref. 152,548k) and serves to prevent a formation of crusts in plants for evaporating spent sulfite liquor, the sodium salt of a polyacrylic acid having the molecular weight 7,000 is added to the liquor in a small quantity so that the formation of crusts will be prevented for a few days.
U.S. Pat. No. 4,255,309 teaches the addition of a processing aid consisting of a polyblend of polyacrylic acid and a copolymer of maleic anhydride and vinyl methyl ether to a black liquor which has been formed in a sulfate process and is being evaporated.
In the process disclosed in U.S. Pat. No. 4,263,092, black liquor is treated in evaporating systems only with polyacrylic acid in order to avoid a deposition of sodium sulfate and sodium carbonate or of a double salt of said compounds. But there will be no deposition of gypsum in the production of sulfate pulp.
The previously known processes have not always adequately satisfied the requirements to be met in commercial practice and it is still necessary in the sulfite pulp industry to provide for a monitoring of deposits in order to maintain the evaporation capacity at a reasonable steam consumption.
In the design of evaporating systems that requirement has already been taken into account in that individual evaporating stages can be shut down and cleaned and are subsequently re-included in the process. In each plant, a predetermined schedule is usually followed in which individual stages are shut down and rinsed with so-called sour condensate for some hours. In addition, a more thorough cleaning, e.g., with nitric acid or sodium hydroxide solution, is required after a certain period of time.